a) Field of the Invention
The present invention relates to a directional microphone, in particular having symmetrical directivity.
b) Description of the Related Art
A directional microphone normally comprises an interference tube which is provided with sound inlets along the length of its tube axis, which are normally covered with acoustic damping material. The interference tube can be closed at its front end by an acoustic absorber, and at its rear end by a pressure microphone or an electroacoustic transducer, free from reflection. A wave is emitted from each sound inlet in the interference tube which, with lateral acoustic irradiation of the interference tube, are not cophasal, i.e., are not found in phase and interfere in that a lobar directional characteristic results. The interference effect with directional microphones disappears with sound wavelengths which are large compared with the length of the interference tube. This happens typically below 1-3 kHz. In this frequency range the interference tube acts independently of direction on the basis of its sound inlets covered with damping material, as an acoustic low pass for the sound which arrives at the forward side of a membrane of the electroacoustic transducer.
So as to also achieve a directional effect for lower frequencies, the interference tube must be combined with a pressure gradient transducer, which likewise has an acoustic low pass on its rear sound inlet. Both low passes act as delay for the sound which arrives at the forward or the rear side of the electroacoustic transducer membrane. These transit times are added to the directional transit times which require the sound at the forward and rear acoustic inlets of the electroacoustic transducer. Without the above filter, a pressure gradient transducer would show its minimal sensitivity with lateral acoustic irradiation. In an ideal situation, the resulting membrane motion would return to zero, as identical pressure would prevail on both sides of the electroacoustic transducer membrane (extinction). The angle of extinction can, for example, be set at 120xc2x0 (supercardiod) or 180xc2x0 (cardiod) through the above-named filter.
A directional effect which is rotation-symmetrical to the interference tube is usually achieved with symmetrically arranged interference tubes and pressure gradient capsules. The interference tube is provided with 2,3 or 4 rows of sound inlet openings and the rear acoustic inlets of the electroacoustic transducer are placed centrally or symmetrical to the center, i.e., the tube axis. Usually the interference tube is protected by a housing, whose sound inlet openings are mostly placed behind a dust proof screen. In-between the housing and the interference tube, a volume or a space is needed which envelopes the interference tube, so that the sound can arrive unhindered at all the interference tube sound inlets. The diameter of the interference tube itself cannot be constructed infinitely small, as otherwise the resistance for the sound waves traveling therein would become too great. Usually an optimal diameter of the interference tube is found which is substantially the same as the membrane diameter of the electroacoustic transducer.
Tubular directional microphones having a small diameter are usually arranged in practice with only a single-sided track of punched holes. Thus space is saved and construction is simpler, as holes are constructed on one side only. Damping material must be precisely applied and also a protective screen. The rear acoustic or rear sound inlets are always realized symmetrically to the tube axis by circulating holes or slits (see FIG. 3a). This is usually necessary if the rear inlet of the electroacoustic transducer is placed on the rear end surface of the transducer and the sound is therefore not directly supplied to the rear side of the membrane. Then problems can arise with the sound supply through the volume or space in the tube behind the electroacoustic transducer due to resonances which are reduced by opening this volume by means of circulating holes or slits.
It has been shown that known tubular directional microphones are either expensive or are relatively large, so that specific operational fields are not available to them. In addition, although smaller tubular directional microphones are known, they have a strongly non-symmetrical directivity (see FIG. 6).
It is the primary object of the present invention to create a directional microphone having symmetrical directivity which is compact and small, and can be carried out in a simple construction.
The above object is met in a directional microphone, having symmetrical directivity, encompassing an interference tube with a tube axis, and a forward and a rear end face, which incorporates an electroacoustic transducer in the inside in the vicinity of the rear end face. Both sides of the electroacoustic transducer has sound inlets. The invention is directed to an improvement comprising that sound inlets are arranged on both sides of the electroacoustic transducer non-symmetrically to the tube axis, referring to the outer periphery of the interference tube. Therefore the suggested solution provides for a directional microphone which is strongly non-symmetrical to its center or the tube axis. Through this a directional microphone is created which is constructed in a space saving manner, and therefore can be particularly slim and inconspicuous. In addition, this directional microphone has a symmetrical directivity which can be achieved in spite of this small and compact construction. Deviations from the symmetry of the directivity can only arise with high frequencies if the sound is not sufficiently bent around the microphone so as to arrive in sufficient quantity at the non-symmetrical sound inputs (shading off through the directional tube itself).
So as to achieve a directivity symmetrical to the tube axis, a plurality of punched hole tracks symmetrically distributed around the circumference of the tube are not necessary. Furthermore, it is not necessary that the track of punched holes is located on the tube axis. With the inventive microphone there is the possibility that a track of punched holes can be located on a smaller diameter as compared with the diameter of the interference tube. Thus it is well possible that the rear sound inlets which lead to the rear inlet of the sound transducer, can be arranged just as non-symmetrically to the tube axis, i.e., which lie in the same plane as the forward inlets, and to construct the interference tube suitably. To achieve a symmetrical directivity of a microphone having forward sound inlets located non-symmetrically to the tube axis, the rear sound inlets can be arranged just as non-symmetrically to the tube axis and do not have to be arranged symmetrically to the tube axis. The invention encompasses the extreme case where the track of punched holes is located on the surface zone area respectively on the axis of the interference tube, and represents the position of a diameter of between 0 and the tube diameter as being particularly suitable.
The non-symmetrical arrangement of the sound inlets to the tube axis opposite the outer circumference of the interference tube can be achieved in differing ways. If the interference tube has a round, i.e., circular cross-section, then this non-symmetry can be achieved for the forward sound inlets, arranged between the forward end of the interference tube and the acoustic transducer, which if necessary can be arranged in a row preferably extending parallely to the tube axis, in that the interference tube is provided with a flattening extending in the direction of the tube axis, into which the row of sound inlets is incorporated.
Thereby the central points of the sound inlets can be respectively unevenly or only partially unevenly spaced from one another. It has been shown to be useful if the central points of the individual sound inlets are arranged within the row of sound inlets, having a distance from one another which corresponds at most to half the shortest wavelength to be transmitted.
Likewise the sound inlets can have identical or differing inlet shapes. Furthermore, it has been proven to be advantageous if the sound inlets possess inlet shapes which are respectively different from one another. There is also, of course, the possibility that the sound inlets possess inlet shapes which only differ partially from one another.
On the other hand the differing inlet shapes can be realized differently. It is particularly advantageous if the sound inlet which is directly adjacent to the end of the interference tube opposite the end face incorporating an electroacoustic transducer, has an oval or elliptic inlet shape having at least a longer main axis arranged approximately parallel to the tube axis, and that the successive sound inlets in the row of sound inlets adjoining this sound inlet have a geometric cross-section shape which is similar to the oval inlet shape, having a reducing main axis in comparison with the longer main axis of this sound inlet, preferably a gradually reducing main axis. Thus the last sound inlets can have a round input shape or a round cross-section, whereby the diameter as the main axis of these round cross-sections can be successively decreased. In this connection it has been further shown to be advantageous if the longer main axis reduces gradually, adjusting to the distance between centers of the sound inlets. Furthermore, in place of a plurality of holes as sound inlets, one or more slits can be provided. It is also favorable if the openings or inlets have low forward acoustic impedance. In this connection larger openings or low impedance dampings can be provided.
Except for the non-symmetry as regards the sound inlets, the interference tube itself can have any desired cross-sectional shape. For example the interference tube can have a cylindrical shape. It has been proved to be particularly advantageous if the interference tube tapers out from the end incorporating the electroacoustic transducer in a truncated cone shape. In addition, it has been proved to be useful if an intermediate space or volume of air is provided between an electroacoustic transducer and the end face of the interference tube which is close to the electroacoustic transducer. This intermediate space is in contact with the surroundings through at least one further sound inlet between the electroacoustic transducer and the rear end face of the interference tube. The further sound inlet can therefore be located in a plane with the sound inlets of the row of sound inlets.
For example, the further sound inlet can be so arranged through a single slit on one side that in the center its opening is in alignment with the row of sound inlets of the interference tube. Through this the row of sound inlets arranged in front of the electroacoustic transducer, and the further sound inlet arranged behind the electroacoustic transducer are located in one line or in one plane. Likewise the further sound inlet can be constructed, for example, through small, if need be, round bores. The operating times from any point in space to all the acoustic inlets of the electroacoustic transducer differ from one another by an amount fixed as a result of construction, which does not change if the directional microphone is rotated about its own axis. In the case where the sound does not arrive directly through the rear acoustic inlets to the rear side of the electroacoustic transducer membrane, rather firstly arrives in a volume behind the electroacoustic transducer and then at the individual acoustic inlets of the electroacoustic transducer, the electroacoustic transducer must be constructed acoustically of particularly high impedance and the volume be constructed particularly small. Both requirements harmonize with the requirementxe2x80x94above allxe2x80x94to keep the directional microphone diameter small. In addition, the rear acoustic inlets of the electroacoustic transducer can be used as additional acoustic filters to influence the ratio of the portions of sound which arrive at both sides of the membrane, in favor of the portion which passes the interference tube for higher frequencies.
The rear end of the interference tube opposite the sound transducer can be either closed at the end face or provided with openings. In the latter case, the acoustically active mass is increased with larger wavelengths and thus the frequency response is extended to lower frequencies.
Furthermore, it can be provided that the sound inlets are covered with a damping material on their outer side. If no further structural element is provided on the outer side of the directional microphone, then the damping material can be so-formulated with an inventive microphone having a flattening and an otherwise round cross-section, as to complete the interference tube cross-section to a round cross-section. The same applies if the sound inlets are covered with a protective screen on their outer side.